Electrophotographic processes and apparatus



March 15, 1966 H. c. MEDLEY ETAL ELECTROPHOTOGRAPHIC PROCESSES ANDAPPARATUS 2 Sheets-Sheet 1 Filed July 28, 1961 12 [V, ii J T 14 20 (0)DIRECT H POTENTIAL o SUPPLY T K \1 FIG 1 36 -J I 24 O 24 v 26'+ +J+ 20'DIRECT POTENTIAL SUPPLY 60 2o DIRECT T POTENTIAL FIG. 3

SUPPLY I I P P EXL F UWLY 59' \@;-4e

- INVENTORJ:

HAROLD C. MEDLEY ROLAND MSCHAFFERT ATTORNEY March 15, 1966 Filed July 281961 H. C. MEDLEY ETAL ELECTROPHOTOGRAPHIC PROCESSES AND APPARATUS 2Sheets-Sheet 2 CAP VOLTAGE BREAKDOWN VOLTAGE (VOLTS) GAP WIDTH (MICRONS)FIG. 2

United States Patent 3,240,596 ELECTROPHOTOGRAPHIC PROCESSES ANDAPPARATUS Harold Clinton Medley, San Jose, Roland Michael Schalfert,Saratoga, (Ialifi, assignors to International Business MachinesCorporation, New York, N.Y., a corporation of New York Filed July 28,1961, Ser. No. 127,725 5 Claims. (Cl. 961) The invention relates toelectrophotography and electrostatic printing, and it particularlypertains to the transfer of latent electrostatic images from anelectrophotographic plate or other medium to dielectric material.

The transfer of latent electrostatic images from one surface to another,as for example from an electrophotographic plate to a dielectricsurface, provides a method of electrostatic printing or copying freefrom the steps of plate and drum cleaning, thereby eliminating the needfor cleaning devices, and consequently improving the life of plates anddrums and reducing the maintenance thereof. However, the prior artprocesses of electrostatic image transfer have not been found suitablefor practical applications of the teaching. Despite some ratherremarkable developments in the field, processes known in the art for thetransfer of electrostatic images (an art at times referred to by theacronym tesi) have not found practical application in commercialelectrophotographic or electrostatic printing as yet. The reasons forthis situation apparently reside in certain critical relationships and/or disadvantages pertinent to the prior art techniques, among which arecritical spacing of surfaces at small but definite distances, criticalvoltage regulation, relatively high voltage operation required, and lowand non-uniform resolution and contrast attained.

It is known to those versed in the art that the inherent resolution andsharpness of electrostatic images, such as those formed by theelectrophotographic process on amorphous selenium plates, are extremelyhigh. However the prior art methods of electrostatic image transferrequire migration of electric charges across a relatively wide air gapwhich, due to image spread, results in a transferred electrostatic imagewith poor resolution and lacking in sharpness. Furthermore, thesepresent methods require maintaining the width of the air gap withinclosely controlled critical limits. This is extremely diificult andalmost impossible to achieve in practice, since variation of a fewmicrons will result in non-uniform charge transfer. Thus, thetransferred image will be non-uniform in strength resulting in densityvariations and generally poor quality of the developed image. Also, theuse of prior art methods results in rather low contrast of thetransferred image, since electric charges are transferred in backgroundareas as well as in image areas, resulting in a charge pattern on thedielectric of the same polarity for both the background and the imagecharges. This condition results in poor contrast with high backgroundwhen the transferred image is developed with fine electroscopicparticles.

An object of the invention is to transfer a latent electrostatic imagefrom an electrostatic image-bearing surface (such as aphotoconductive-insulating surface) to a dielectric surface while thesesurfaces are in virtual contact, thereby attaining greater accuracy ofreproduction of the original electrostatic image with a high degree ofresolution and sharpness, and eliminating the necessity of maintainingan air gap of critical dimensions.

Another object of the invention is to transfer a latent electrostaticimage to a dielectric surface whereby the resultant image areas on thedielectric surface are of one polarity of electric charge and thebackground areas are Patented Mar. 15, 1966 of the opposite polarity ofelectric charge, thus producing greater contrast and cleaner developmentof the image.

Another object of the invention is to provide a method for electrostaticimage transfer resulting in a stable latent image-bearing medium whichcan be developed at a station removed from the immediate vicinity of theimage transfer station.

A further object of the invention is to provide methods and apparatusfor latent electrostatic image transfer where developing can be doneunder full illumination and under continuous visual inspection.

Still another object of the invention is to provide a record mediumchemically and physically stable, unaffected by light or aging, andadaptable for spot processing so that variable data of a record may bechanged from time to time without also reproducing the constant data.

A still further object of the invention is to provide a process oftemporary electrostatic image transfer useful in producing images forprojection display or for a pluralstep image reduction process utilizinglow sensitivity dry processing film.

A still further object of the invention is to develop a process fortransferring an electrostatic image to a hard copy medium, such as papercoated with dielectric film.

According to the invention, latent electrostatic images are produced ondielectric material by electric charge transfer from surfaces ofphotoconductive insulating material or another dielectric material uponwhich electrostatic images have been formed by known techniques. In oneknown method of forming the initial latent electrostatic image, aphotoconductive insulating layer is sensitized in darkness byelectrically charging the surface uniformly to a predetermined potentialof given polarity with respect to the conductive surface backingelement, and an image of the desired subject is optically projected ontothe photoconductive insulating layer to produce a charge patterncorresponding to the image of the desired subject. Other methods offorming initial electrostatic images are known, such as pulsing shapedcharacters located closely adjacent to a dielectric surface with highvoltage. However, the manner in which the initial electrostatic image isformed is not a part in itself of the invention.

According to the invention, the layer of dielectric material to whichthe electrostatic image is to be transferred is backed with a conductiveelement and the dielectric surface electrically charged uniformly to apotential of polarity opposite to the polarity of the charge of theinitial electrostatic image. The charge pattern is then transferred byplacing the charged layer of dielectric material in contact with thesurface layer containing the initial electrostatic image in darkness,and applying a direct potential from an external source between the twoconductive backings of the respective layers. The potential is appliedbetween the conductive backing elements such that the lead attached tothe conductive backing of the dielectric layer is of the same polarityas the charge originally placed on the dielectric layer.

The direct potential applied between the conductive surface elementsafter the materials have been placed in contact is preferably obtainedfrom a variable source, so that it can be adjusted from zero to arelatively high value, up to several thousand volts.

Apparatus for automatically and continuously carrying out a processaccording to the invention comprises in one form a rotatable drum havinga peripheral conductive surface element over which there is a layer ofphotoconductive material, conventional means for uniformly charging thephotoconductive layer at given polarity to a predetermined potential ofgiven polarity with respect to the conductive element, conventionalmeans for projecting an optical image on the charged layer to create alatent electrostatic charge pattern on the layer. Another drum having aperipheral conductive surface over which a dielectric film material islaid, provides the conductive surface backing for the dielectric filmmaterial necessary during the processing. Conventional means areemployed for charging the surface of the dielectric film materialsubstantially to a predetermined potential of polarity opposite to thepolarity of the charge on the layer of photoconductive material. Theconductive surface elements of the drums electrically isolated andconnected to opposite terminals of a charge transfer aiding potentialsource in accordance with the invention which is adjustable from zero toseveral thousand volts direct potential. The apparatus also comprisesconventional means for discharging the photoconductive layer aftertransfer of the image therefrom to the dielectric film material andconventional means for developing the image transferred to thedielectric material.

In order that full advantage of the invention may be readily obtained inpractice, a preferred embodiment thereof is described in detailhereinafter with reference to the accompanying drawings forming a partof the specification, and in which:

FIG. 1, sections (a), (b), (c) and (d) illustrate apparatus as arrangedfor particular steps in the transfer of an electrostatic image accordingto the invention;

FIG. 2 is a graph providing data and indicating conditions important inan understanding of the invention; and

FIG. 3 illustrates apparatus for carrying out the process of theinvention in an automatic and continuous mode of operation.

FIG. 1 depicts the essentials of apparatus necessary for carrying out amethod according to the invention for forming a latent electrostaticimage on dielectric material. An electrostatic image, corresponding to adesired document is formed by conventional means on a known medium, forexample, a xerographic plate comprising a conductive substrate 12 coatedwith photoconducting material 14 such as amorphous selenium. Aconventional corona charging unit 16, energized by electric connectionsmade through a polarity reversing switch 18 to a direct potential supply20 capable of delivering between 4000 and 9000 volts, is swept acrossthe surface of the selenium layer 14 to place a uniform positive chargethereon as shown in FIG. 1(a). This must be done in darkness asotherwise any light striking the selenium layer 14 will discharge theelectric charge laid down. As shown in FIG. 1(b) the image of a document22, illuminated by photo flood lamps 24 or other suitable photographicilluminating lamps, is projected by means of a lens system suggested bya symbolic lens 26 onto the xerographic plate 10 within someconventional arrangement (not shown) for excluding ambient light. Thelight areas 28 of the document 22 are projected on to the selenium layer14 discharging the positive charge immediately there above and leavingthe positive charges only in the areas corresponding to dark areas ofthe document 22.

Referring again to FIG. 1(a), a desired dielectric material 30 is backedby a conducting backing surface element 32 and sensitized by chargingnegatively to a uniform potential by sweeping with another conventionalcorona charging unit 36 connected through the electric reversing switch18 to the direct potential supply 20. According to the invention thecharge placed on the dielectric material 30 is always of oppositepolarity to the charge placed on the plate 10. In the arrangement shownamorphous selenium is given as an example of photoconductive material.While amorphous selenium can be charged negatively and electrostaticimage transfer accomplishcd wording to the invention, his well knownthat this material functions best when positively charged. It should beunderstood, however, that it is clearly within the scope of theinvention to form an electrostatic image with either polarity andtransfer the same to a dielectric material charged to the oppositepolarity.

As shown in FIG. 1(a), the image record plate 10 which has been chargedin accordance with the desired image is superimposed in darkness overthe dielectric material 30 with conductive backing surface element 32 inplace. The backed selenium 14 of the plate 10 and the backed dielectricmaterial 30 are brought into contact without any direct electricconnection between the conductive backing elements 12 and 32. Theconductive backing elements 12 and 32 are then electricallyinterconnected, as by throwing an electric transfer switch 38. The imageis transferred according to the invention when the conductive backingelements 12 and 32 are maintained at a predetermined direct potentialobtained from a transfer aiding potential supply 39 capable of supplyingdirect potential in a range from zero to several thousand volts as willbe described hereinafter. In the special case of interconnection at zeropotential the switch 38 may be thrown to a contact bypassing the supply39, if desired.

It is obvious, of course, that these methods can also be used totransfer images from negatively charged electrostatic images, in whichcase the dielectric material would be charged to a positive polarity.Most dielectric materials function about equally well with eitherpolarity although polyethylene functions better with negative chargingthan with positive charging.

Laboratory tests of the above described methods have indicated that thistechnique is less sensitive to surface defects than prior knowntechniques of electrostatic image transfer.

Examples of dielectric material which have been used successfully:Polyethylene glycol terephthalate, which is a polyester most commonlyknown by the registered trademark Mylar; polystyrene; polyethylene;styrene butadiene copolymers. Aluminized Mylar film has been used as amaterial having a conductive backing surface integral with thedielectric with and without additional coating; aluminum plates; andconductive glass slides more commonly known by the registered trademarkNESA have also been used for conductive backing surfaces.

The mechanism of transfer of electrostatic images from one surface toanother has been explained on the basis of gaseous discharge phenomena.In practice, the gas usually will be air at atmospheric pressure. Thisgas, or air, fills the gap between the electrostatic image surface andthe surface of the dielectric material to which the image is to betransferred. As the two surfaces are brought together, the gap betweendecreases from a relatively large valve to a very thin film even whenthe surfaces are brought into virtual contact. The gas film thickness atcontact will depend upon the degree of surface smoothness. Surfacespolished to optical standards and placed in contact are still separatedby gaps of the order of 3 X 10 cm. The gas film between a smoothamorphous selenium surface in contact with a smoother dielectric film isestimated to be in the range of 0.5 to 1.0 micron.

V Paschens law for the breakdown of gas in an electric field states thatthe breakdown voltage in a linear function of the product of gaspressure and distance between the electrodes. This law holds for valuesfor the distancepressure product which are greater than about 5 mm. Xmm. of mercury in air. Below this value, the breakdown voltage increasessharply for gases at low pressures. However, for relatively highpressures (for example, normal atmospheric pressure), the sharp upwardturn of the breakdown curve is not observed. At these relatively highpressures, the gaps (below the Paschen minimum) p ating the surfaces arevery small, and, discharge: is

due primarily to field emission because of the very high electricfields. For instance, the breakdown curve in air for very small gaps(less than about 8 microns for air at 760 mm. of mercury) is a downwardsloping curve due to electron emission from the surfaces forming thegap. Such a curve is very useful in practice when it is desired totransfer charges across extremely small gaps.

It will be appreciated that charge transfer is preferred across a smallgap to avoid the image spread which occurs when the charges aretransferred across a relatively Wide gap. In all of the techniques ofelectrostatic image transfer, the image surface and the dielectricsurface are brought together and then, subsequently, separated. Duringthese manipulations it is desired that conditions be avoided thatproduce air breakdown discharges when the gap between the surface islarge. Spark discharges are to be avoided under any conditions andaccording to the invention, transfer is made under conditions that willproduce silent discharge when the surfaces are very close together;generally less than 3 microns apart. Preferably according to theinvention while the surfaces are in contact, conditions are altered, asby closing the transfer switch 38, to provide a voltage across the gapwhich will produce discharge by field emission.

Reference is now made to FIG. 2 in which the heavy solid black curve Mrepresents the minimum voltage for discharge as a function of gap widthfor air at atmospheric pressure. The nature of this curve M will bedifferent for different gases and different pressures; however, theslope of the critical field emission line will be independent of thetype of gas, depending only on the nature of the surfaces forming thegap. The plateau portion of the curve (at approximately 360 volts) is anextension of the Paschen minimum; the upward sloping line to the rightof the plateau which is the lower portion of the normal Paschen curvefor breakdown of the gaseous discharge; and the steep sloping lineportion to the left of the plateau is the critical field emission line,which in FIG. 2 has been drawn to correspond to a slope of 10 volts percm. Shown also in FIG. 2 are curves representing gap voltage versus gapwidth for several different conditions of applied voltage, chargevoltage of the initial electrostatic image, charge voltage of thedielectric surface, and thickness and dielectric constant of thedielectric materials. The light solid curves A, B, C, and D indicate gapvoltages in the regions of the electrostatic images and the the lightbroken curves A, B, C, and D are for gap voltages in the regions of thebackground areas.

Several examples of image transfer according to the invention will nowbe described in order to more clearly set forth a manner of practicingthe invention.

Example 1 Applied voltage volts 1500 Charge voltage of electrostaticimage do +500 Charge voltage on the dielectric layer do +1000 Thicknessof image layer 14 microns 12.6 Thickness of dielectric layer 30 do 36Dielectric constant of layer 14 6.3 Dielectric constant of layer 30 3.0

These conditions are illustrated by curves A and A in FIG. 2. It will benoted that both curves A and A are above the critical field emissionline portions of the curve M. Thus, electric charges will transfer whilethe surfaces of layers 14 and 30 are in virtual contact. Since the curveA for the image area is higher than that of curve A for the backgroundarea, a greater amount of charge will transfer in the image area than inthe background.

A process for accomplishing image transfer under the conditions setforth above is as follows:

A photoconductive layer 14 is given a positive surface charge of 500volts with respect to the conductive base 12 as shown in FIG. 1(a). Anelectrostatic image is produced in the layer 14 as shown in FIG. 1(b).The dielectric layer 30 is then given a negative surface charge of 1000volts with respect to the conductive base 32 as shown in FIG. 1(a).Layers 30 and 14 are then brought into virtual contact and the switch 38in FIG. 1(c), is connected momentarily to the positive pole of a 1500volt direct potential source. The switch 38 is then disconnected and thetwo surfaces of the layers 14 and 38 are separated.

It is found after the above procedure, that an electrostatic chargepattern corresponding to the initial image has been formed on thedielectric layer such that the image and background charges are ofopposite polarity as shown in FIG. 1(d) the image areas having apositive polarity charge of approximately 260 volts, and the backgroundareas having a negative polarity charge of approximately volts. Thiscondition is particularly desirable for attaining high contrast indevelopment, for example, with a developing technique such as the powdercloud method where the image surface is subjected to the cloud ofaerosol particles electrically charged with negative polarity.

Furthermore, it will be noted that charge transfer took place while thesurfaces were in virtual contact with the gap width in the range of 0.5to 1.0 micron. Thus, this procedure is capable of achieving highelectrostatic resolution in the transferred image, and there is verylittle loss of resolution during the transfer step. It is known that themaximum resolution attainable in electrostatic image transfer is aninverse function of the gap width, being approximately proportional tothe reciprocal of the gap width. It has been found that when transfertakes place across a gap width of one micron, a maximum resolution of200 lines per mm. is possible.

Example 2 Applied voltage volts +1000 Charge voltage of image do +700Charge voltage on dielectric do 300 Thickness of dielectric layer 14microns 25 Thickness of dielectric layer 30 do 25 Dielectric constant oflayer 14 6.3 Dielectric constant of layer 30 3.0

These conditions are represented by the curves B and B in FIG. 2. Itwill be noted that as in the case of Example 1, the image charge istransferred by field emission while the surfaces are in virtual contact.Thus, a high maximum resolution is attainable in the transferred image.It will also be noted from curve B that only a small amount of charge istransferred in the back'gnound areas under these conditions.

The procedure for transferring electrostatic images with the conditionsof Example 2 is essentially the same as for Example 1 except that thevoltages applied are of different values.

It is found that after separation of the surfaces, an electrostaticimage corresponding to the initial image is formed on the dielectriclayer 30, such that the image areas are charged to a positive polarityof approximately volts, and the background areas are charged to anegative polarity of approximately 275 volts.

Example 3 Applied voltage 0 Charge voltage of image volts +800 Chargevoltage of dielectric do 200 Thickness of layer 14 microns 25 Thicknessof layer 30 do 25 Dielectric constant of layer 14 6.3 Dielectricconstant of layer 30 3.0

These conditions give rise to curve C and C in FIG. 2. It will be notedthat with these conditions charge will transfer in the image areas bygaseous discharge at about 6.7 microns whereas no discharge will takeplace in the background areas as can be seen from curve C, which curve Cdoes not cross the minimum curve M.

The process for transferring electrostatic images under these conditionsis somewhat different from the process used in Examples 1 and 2. In thiscase, the image layer 14 is charged positively to 800 volts and anelectrostatic image formed on this layer in the usual manner. A negativecharge of 200 volts is then applied to the dielectric layer 30. Thesurfaces are then brought together, and switch 38 is connected to theright-hand terminal of FIG. 1(0) to electrically interconnect theconductive backing surfaces 12 and 32 at substantially zero potential,and preferably at ground potential as shown. This connection ismaintained as the surfaces are separated.

It is found that with this procedure and these conditions anelectrostatic image is formed on the layer 30 such that the image areasare charged to a positive polarity of 195 volts, and the backgroundremains charged to a negative polarity of 200 volts.

Since transfer takes place at 6.7 microns, the maximum resolutionattainable for electrostatic image is transferred in this manner isabout 30 lines per mm.

Example 4 Applied voltage Charge voltage of the image 'volts +700 Chargevoltage of the dielectric layer 30 do 500 Thickness of layer 14 microns50.4 Thickness of layer 30 do 48.0 Dielectric constant of layer 14 6.3Dielectric constant of layer 30 3.0

These conditions result in curves D and D' in FIG. 2, from which it willbe noted that transfer in the image areas takes place by gaseousdischarge at a gap width of 11.0 microns, whereas no charge istransferred in background areas.

The procedure here is the same as Example 3, except that the voltagesapplied to the surfaces are different. After separating the surfaces itis found that an electrostatic image has been formed on the dielectriclayer 30 such that the image areas are essentially neutralized whereasthe background areas remain charged to a negative polarity of 500 volts.The maximum resolution attainable in this case is about 18 lines per mm.

Examples 1 and 2 are particularly suited for transferring electrostaticimages of micro-image size, whereas Examples 3 and 4 are suitable fortransferring images of intermediate or macro-image size, for example,images normally readable without optical aids.

The above examples all utilize initial electrostatic images of positivepolarity. It will be appreciated that images of negative polarity couldbe transferred by similar procedures and processes, in which case thedielectric layer 30 would be charged to positive polarity, instead ofnegative as shown in and described hereinbefore.

Thus far the techniques of the invention have been described as astepwise process performed with flat surface structural elements.Continuous processing, using an image retaining drum instead of theelectrophotographic plate, is possible with the techniques and one sucharrangement according to this invention is shown in FIG. 3. Theessentials are shown in this illustration, it being understood thatconventional methods and structures for transporting the variouscomponents of :the apparatus, shielding the charged areas from light orelectrostatic fields, and the like, are readily apparent to thoseskilled in the art.

A drum conveniently completely metallic but at least having a conductiveperipheral surface element 12' maintained at ground potential, as shown,and having a charge image recording layer, for example, of amorphousselenium, 14' thereon is arranged in a light tight housing 40, the upperpart of which is hinged so that it may be 8 opened wide for access infeeding documents and servicing the unit.

Images on a continuous web of documents 22' (or on single documentsinserted and removed by hand, one after the other, into a feeding slot42), illuminated by a synchronized slit exposure system indicated onlyschematically by a pair of lamps 24' are produced in succession ascharge images on the selenium drum 10' in more or less conventionalmanner. A web 30 of dielectric material unwinding from a feed reel 44and winding on a takeup reel 46 is carried over guide rollers asnecessary. At a point where the web 30 passes over a conductive drum 47which is maintained at fixed reference potential, preferably ground, theweb is given a uniform negative charge by means of a corona chargingunit 36'. A pair of direct potential supplies 20', 20 are arranged toenergize the corona charging units 16 and 36' with potentials ofopposite polarity. A suitably housed lamp 60 is arranged to dischargethe image charge remaining on the selenium coating 14' after chargetransfer.

Insulating rollers 48 urge the web 30 into contact with the xerographicdrum 10. Another drum 49 is arranged to back the web 30' at the point onthe xerographic drum 10 where it is desired that the charge transferprocess take place. This backing drum 49 has at least a peripheralconductive surface element 32' forming the backing conductive surfaceelement of the dielectric web 30 during the transfer process andpreferably connected to the shaft on which the backing drum 49 rotatesas shown. The conductive surface element 32 on the backing drum 49 iselectrically insulated from the remainder of the structure andconnected, as exemplified by the electric charge transfer switch 38, toan aiding potential source 39', delivering from zero to several thousandvolts direct potential. In the special case wherein it is desired towork at zero potential between the interconnected backing surfaces, asexemplified by the electric switch 38' in the right-hand position, thebacking surface element 32 is at ground potential. More often it will bedesired to use transfer aiding potential in which case the connection inFIG. 3 would be as exemplified by the electric switch 38 being thrown tothe left-hand position. It should be understood that the electric switch38' serves only to connect the aiding potential supply for the mode ofoperation desired; the switching during the charge transfer processbeing effected by the translation of the web 30' with respect to thedrums 10 and 49.

The resulting charge pattern is developed by a conventional developingmeans; shown as a powder charge unit 50 comprising a powder 51 in ahopper 52 cascading down onto the web 30 as at point 53. The overflowpowder is caught in a bin 54 by suitable arrangement (not shown)returned to the hopper 51 for later use. The image developed on the web30' is then fixed by means of a heat fusing unit 58 according to knowntechniques.

By suitable arrangement, the lens 26 can be shuttered; the coronadischarge unit 16 can be disconnected; and the discharge lamp 60 can beextinguished, so that the residual charge on the selenium layer 14' canbe used to transfer additional images as desired. Conceivably thearrangement can be so shuttered in known fashion that an image on theselenium coating 14' can be discharged in a local area only and a newportion inserted thereat for spot updating of the information recorded.

The invention claimed is:

'1. A method of transferring an electrostatic image on a photoconductiveinsulator material to dielectric material,

0 comprising the steps of placing a uniform charge on one face of saiddielectric material of polarity opposite to that on one face formingsaid electrostatic image on said insulator material,

placing said one face of said charged dielectric material in virtualcontact with said one face of said charged insulator material,electrically interconnecting said dielectric material and said insulatormaterial at a predetermined direct potential difference for electronflow therebetween,

disconnecting said electric connection between said materials, and

separating said materials,

thereby transferring said image to said dielectric material with theimage thereon at one polarity and the background at opposite polarity.

2. A method of transfering an electrostatic image on a face ofphotoconductive material backed by a conductive surface element todielectric material, comprising the steps of backing said dielectricmaterial with a conductive surface element,

placing a uniform charge on the face of said dielectric material ofpolarity opposite to that forming said electrostatic image on the faceof said insulator material,

placing said face of said charged dielectric material in virtual contactwith said face of said charged insulator material,

electrically interconnecting said backing surface elements at apredetermined direct potential difference, disconnecting said electricconnection between said elements, and

separating said materials,

thereby transferring said image to said dielectric material with theimage thereon at one polarity and the background at opposte polarity.

3. A method of transferring an electrostatic image on a face ofphotoconductive insulator material backed by a conductive surfaceelement to dielectric material, comprising the steps of backing saiddielectric material with a conductive surface element,

placing a uniform charge on the face of said dielectric material ofpolarity opposite to that forming said electrostatic image on the faceof said insulator material,

placing said face of said charged dielectric material in virtual contactwith said face of said charged insulator material,

electrically interconnecting said backing surface ele-.

ments at zero direct potential difference, disconnecting said electricconnection between said elements, and

separating said materials,

thereby transferring said image to said dielectric material with theimage at one polarity and the background at opposite polarity.

4. A method of transferring an electrostatic image on a face ofphotoconductive insulator material backed by a conductive surfaceelement to dielectric material, comprising the steps of backing saiddielectric material with a conductive surface element,

placing a uniform charge on the face of said dielectric material ofpolarity opposite to that forming said electrostatic image on saidinsulator material, placing said face of said charged dielectricmaterial in virtual contact with said face of said charged insulatormaterial, electrically interconnecting said backing surface elements ata predetermined direct potential difference of substantially less thanfive hundred volts, disconnecting said electric connection between saidelements, and separating the said materials, thereby transferring saidimage to said dielectric material with the image at one polarity and thebackground at a polarity opposite to that of said image. 5. A method oftransferring an electrostatic image ona face of photoconductive materialbacked by a conductive surface element to dielectric material,comprising the steps of backing said dielectric material with aconductive surface element, placing a uniform charge on the face of saiddielectric material of polarity opposite to that forming sa delectrostatic image on said insulator material, placing said face ofsaid charged dielectric material in virtual contact with said face ofsaid charged insulator material, electrically interconnecting saidbacking surface elements at a predetermined direct potential differenceranging between zero and five hundred volts, disconnecting said electricconnection between said elements, and separating said materials, therebytransferring said image to said dielectric material with the imagerepresented by charge of one polarity opposite to that of thebackground.

References Cited by the Examiner UNITED STATES PATENTS 2,909,971 10/1959Barber 961.7 2,937,943 5/1960 Walkup 96-1 2,946,682 7/1960 Lauriello 9612,947,625 8/1960 Bertelsen 961 2,968,553 1/1961 Gundlach 96-1 2,982,6475/1961 Carlson et al. 96-1 2,984,163 5/1961 Giaimo 1.7 3,062,110 11/1962Shepardson et al. 951.7 3,147,679 9/1964 Schafrert 95-1.7

FOREIGN PATENTS 607,290 10/1960 Canada. 807,079 1/ 1959 Great Britain.855,727 12/1960 Great Britain.

NORMAN G. TORCHIN, Primary Examiner.

HAROLD N. BURSTEIN, Examiner.

1. A METHOD OF TRANSFERRING AN ELECTROSTATIC IMAGE ON A PHOTOCONDUCTIVEINSULATOR MATERIAL TO DIELECTRIC MATERIAL, COMPRISING THE STEPS OFPLACING A UNIFORM CHARGE ON ONE FACE OF SAID DIELECTRIC MATERIAL OFPOLARITY OPPOSITE TO THAT ON ONE FACE FORMING SAID ELECTROSTATIC IMAGEON SAID INSULATOR MATERIAL. PLACING SAID ONE FACE OF SAID CHARGEDDIELECTRIC MATERIAL IN VIRTUAL CONTACT WITH SAID ONE FACE OF SAIDCHARGED INSULATOR MATERIAL, ELECTRICALLY INTERCONNECTING SAID DIELECTRICMATERIAL AND SAID INSULATOR MATERIAL AT A PREDETERMINED DIRECT POTENTIALDIFFERENCE FOR ELECTRON FLOW THEREBETWEEN, DISCONNECTING SAID ELECTRICCONNECTION BETWEEN SAID MATERIALS, AND SEPARATING SAID MATERIALS,THEREBY TRANSFERRING SAID IMAGE TO SAID DIELECTRIC MATERIAL WITH THEIMAGE THEREON AT ONE POLARITY AND THE BACKGROUND AT OPPOSITE POLARITY.